Migration and Movements

The dispersal of East Pacific green turtle hatchlings from natal beaches has not been studied, but it can be assumed to include passive transport by ocean currents over vast distances. According to tag-recovery data (as summarized by Alvarado and Figueroa 1990), East Pacific green turtle migrations occur between the northern and southern extremes of their range. Recoveries of nesting females tagged on the beaches of Michoacán have been documented from El Salvador, Guatemala, Nicaragua, Costa Rica and Colombia. Recoveries have also been documented from Mexican waters, primarily from the Gulf of California and adjacent waters, and from the coast of Oaxaca. Tag recoveries in Central America are most common from El Salvador and Guatemala. In Mexico the frequency of recovery is highest in the Gulf of California and adjacent waters. Of 37 documented recaptures during 1989-1990, 32 were incidental catches, mainly by shrimp and fish trawlers. Most of the recoveries were restricted to coastal waters, perhaps because most commercial fishing in the east Pacific occurs on the narrow continental shelf. The average depth at 13 coastal capture sites was 24.3 ± 5.8 m (range 10.0 - 72.0 m). The longest distance covered by an East Pacific green turtle prior to capture was 3,160 km (measured in the direct line along the coast) by a turtle tagged in Michoacán and recovered at El Faro, Charambira, Colombia in October 1986. Minimum average traveling speed is 22.5 km/day (range 8.0 - 38.0, n=7). A post-nesting female that was satellite-tagged at Colola in Michoacán in October 1991 traveled to Central America and was tracked for two months; she swam about 2,000 km with a daily average distance of 33 km (Byles et al. 1995).

Tag-recovery data indicates that at least part of the East Pacific green turtle population breeding in the Galapagos Islands is recruited from distant feeding grounds. Galapagos-tagged turtles have been recovered in the coastal waters of Costa Rica, Panama, mainland Ecuador, Colombia and Peru (Green 1984; MacFarland 1984). The Galapagos archipelago lies approximately 1,000 km off the coast of mainland Ecuador; thus, movements between mainland feeding and island breeding grounds involve crossing a vast expanse of open ocean. An undetermined portion of the Galapagos breeding contingent remains in the feeding grounds around the Galapagos Islands year-round (Green and Ortiz-Crespo 1982; MacFarland 1984). Some East Pacific green turtles nesting in Costa Rica may also be year-around residents (Cornelius 1986).

Data collected during tuna fishing cruises suggest that East Pacific green turtles are most frequent along a North-South band from 15EN to 5ES along 90EW from January through March, but most frequent between the Galapagos Islands and the Central American coast from July through September (IATTC, unpubl. data). Green (1984) suggested earlier that this observed shift might indicate migratory movement. IATTC data suggest that East Pacific greens are rare near the Mexican coast, and are only present during October through December.

A small aggregation of turtles in San Diego Bay, tentatively identified as East Pacific greens, seems to be year-round residents, and the fact that small (<55 cm SCL) juveniles are regularly seen suggests that turtles are continuing to migrate into the bay (Stinson 1984; McDonald and Dutton 1993; McDonald et al. 1995).

Foraging Biology and Diet

Although East Pacific green turtle feeding grounds are not clearly delimited, the main sites appear to be the west coast of Baja California, Mexico (Scammon’s Lagoon, Tortugas Bay and Magdalena Bay) (Cliffton et al. 1982), the Gulf of California (Felger et al. 1976; Márquez 1990), the Superior and Inferior lagoons in Oaxaca, Mexico (R. Márquez, Instituto Nacional de Pesca, INP, pers. comm., 1989), the Galapagos Islands (Green and Ortiz-Crespo 1982; MacFarland 1984), the Gulf of Fonseca (Honduras), and the Paracas Peninsula in Peru (Márquez 1990). According to tag-recovery information, the feeding grounds of the Michoacán breeding population are restricted to Mexico and Central America (Alvarado and Figueroa 1990), whereas the population breeding in the Galapagos Islands forages from Costa Rica south to Peru (Green and Ortiz-Crespo 1982; Green 1984; MacFarland 1984). It is not known where turtles traveling along the west coast of the United States forage; turtles in San Diego Bay feed on eelgrass and algae in the bay (Dutton and McDonald 1990a,b; McDonald et al. 1995).

Adult East Pacific green turtles are primarily herbivorous, eating sea grasses and algae, and in some areas they may feed on a variety of marine animals. Food items vary among feeding grounds. In Peru the following food items have been reported in stomach content analysis: plants (Macrocystis, Rhodymenia and Gigartina), molluscs (Nassarius, Mytilus and Semele), polychaetes, jellyfish, amphipods, and fish (sardine and anchovy) (Hays-Brown and Brown 1982). In the Galapagos Islands the following items have been reported: algae (Caulerpa, Ulva) and mangrove leaves (Rhizophora mangle) (Pritchard 1971). In Ecuador, Fritts (1981) reported fish eggs in the stomach of a female turtle. In a sample of 19 turtles from Bahia de los Angeles, Gulf of California, an average of 1,230 cm3 of food per individual was obtained, composed of 90.0% algae, 1.0% animal food, and 9.0% unidentified material (Márquez 1990). The algae found in this study were: Gracillaria, Rhodimenia, Gelidium, Grateloupia, Gigartina, Griffitsia, Sargassum, Padina, Ulva, and Cladophora. Sargassum and Gracillaria were the most abundant. In the same study, animal food items included small quantities of small molluscs, crustaceans, bryozoans, sponges, jellyfishes and echinoderms.

Casas-Andrew and Gomez-Aguirre (1980) report similar findings from off the central western coast of Mexico, with Ulva being the most abundant algae in the samples. The stomach contents of one turtle from this study consisted exclusively of the pelagic tunicate Pyrosoma. In the Infiernillo Channel (area between Tiburon Island and the mainland) in the Gulf of California, East Pacific green turtles feed on eelgrass, Zostera marina, and the sea slug, Aplesia californica (Felger and Moser 1987). Feeding habits of hatchlings and juveniles are unknown.

Growth

In the Galapagos Islands, Green (1994) found a mean growth rate of 0.40 to 0.45 cm per year for juveniles 40 - 60 cm SCL, while subadults (60.0 - 66.7 SCL) grew 0.15 cm per year. Growth recorded for turtles in San Diego Bay was considerably faster. Growth rates for two individuals (SCL 54.4 and 46.7 cm) were 6.7 and 5.1 cm/yr, respectively, while an 86.7 cm female grew 3.9 cm in one year (McDonald et al. 1995). Green (1994) reported that since the minimum size of nesting turtles in the Galapagos Islands is 66.7 cm SCL, it may take some turtles at least 50 years to reach sexual maturity. Based on growth rates observed in wild turtles, age at first reproduction (minimum 81.0 cm SCL) for green turtles in the Hawaiian archipelago is estimated to be roughly 10-50 years depending on the feeding ground (Balazs 1982).

Reproduction

Reproduction is seasonal. In most cases gravid females migrate long distances between foraging and breeding grounds (see Migration and Movements). An exception may be the Galapagos Islands, where large numbers of East Pacific green turtles were observed copulating early in the year (IATTC, unpubl. data), and turtles are seen throughout the year. The nesting season varies with location. Nesting occurs in Michoacán between August and January, with a peak in October-November (Alvarado et al. 1985), between March and July at Socorro and Clarion islands (Márquez 1990), between December and May with a peak in February-March on the Galapagos Islands (Green and Ortiz-Crespo 1982), and possibly year-round with a peak in October-March at Playa Naranjo, Costa Rica (Cornelius 1986). On beaches shared with other nesting turtle species (e.g., Colola and Maruata in Michoacán) East Pacific green turtle nesting occurs after the nesting peak of the olive ridley and before that of the leatherback (Alvarado et al. 1985). This may reduce competition for nesting space.

In Michoacán, females typically nest in two or three year cycles and deposit between one and seven clutches per season at about 12-14 day intervals (Alvarado and Figueroa 1990). In the Galapagos Islands, females typically nest in three or five year cycles and deposit between one and five clutches per season at about 14 day intervals (Hurtado 1984). In Playa Naranjo, Costa Rica, females may nest in consecutive years and deposit at least two and perhaps as many as six nests per season at about 14 day intervals (Cornelius 1986).

Average clutch size varies geographically. In Michoacán, the average is 65 eggs (range 1-130, n=916 nests) (Alvarado and Figueroa 1990). In the Galapagos Islands, average clutch size is 84 eggs (range 56-152, n=30 nests) (Hurtado 1984). In Playa Naranjo, Costa Rica, average clutch size is 87 eggs (range 65-107, n=10 nests) (Cornelius 1976). After 42 to 62 days of incubation (Márquez 1990) hatchlings emerge mostly at night and travel quickly to the sea. Nest temperature during incubation influences the sex of hatchlings. At Michoacán, 47 East Pacific green turtle clutches were monitored in 1984 and 1985 to determine the sex ratio of emergent hatchlings. Average temperatures <27.0ec (range="" 26.4-27.0ec)="" during="" the="" mid-third="" of="" incubation="" resulted="" in="" 100%="" males;="" average="" temperatures="" between="" 27.5-31.0ec="" a="" mixed="" sex="" ratio,="" and="" those="">31.0EC (range 31.0-32.9EC) produced 100% females (Alvarado and Figueroa 1987).

In most studied populations of sea turtles (all species), mating does not appear to occur once nesting has commenced. This is true for Chelonia mydas in Australia (Booth and Peters 1972) and Hawaii (Balazs 1980). In contrast, East Pacific green turtle mating apparently can occur both prior to and between nestings at the Michoacán rookery, and sequential mating throughout the season is implied (Alvarado and Figueroa 1991b).

Offshore Behavior

During observations from tuna fishing cruises, East Pacific green turtles are often seen basking at the surface. Turtles seem to be most active around midday; 30% of the green turtles seen swimming were seen around noon (IATTC, unpubl. data). They were almost always seen near islands, feeding very close to the coast. Unlike olive ridleys, East Pacific greens are very rarely seen associated with floating objects. Turtles are often reported in association with fish such as dorado and sharks, but it is not always clear whether these are olive ridleys or East Pacific greens. Although East Pacific greens usually occur singly, they are frequently seen in large groups, usually near the Galapagos Islands (e.g., a group of 59 was seen in July 1991). Sampling from large groups always yielded only mature females (IATTC, unpubl. data).

Health Status

Disease in East Pacific green turtle populations has not been studied. McDonald and Dutton (1990) found early stages of what appear to be fibropapillomas (tumor disease) in several individuals of the San Diego Bay population. Green turtles residing in certain benthic habitats are afflicted by lobulated tumors (fibropapillomas) on their skin, scales, scutes, eyes, oral cavities, and viscera (Balazs and Pooley 1991). The tumors begin as small, localized lesions that rapidly grow to exceed 30 cm in diameter, greatly interfering with or even prohibiting swimming, feeding, breathing, or seeing. The lesions have been classified as fibropapillomas, based on established histologic criteria for tumor classification. The cause of this disease is unknown, but a herpes virus is highly suspected based on recent research (Herbst 1994). The disease has increased to epidemic proportions in Hawaii since the mid-1980s. Similar severe outbreaks in green turtles over the same time period have also been reported in Florida, several Caribbean nations, and at a few other sites worldwide. The extent of the presence of this disease has yet to be identified for the East Pacific green turtle.

Massive East Pacific green turtle mortalities, not obviously connected to human activities, have been reported along the Pacific coasts of Colombia and Costa Rica. During the fall of 1972, 73 moribund sub-adult East Pacific green turtles were observed on Nancite and Naranjo beaches in Costa Rica. No wounds were evident in the affected turtles; however, they exhibited gastro-intestinal disorders (Cornelius 1975). In February 1990, a die-off of about 200 adult female East Pacific green turtles was reported in the northern Pacific coast of Colombia. No wounds or traumas were apparent. It has been suggested that a contagious disease or drowning by trawlers may have caused the mortality (D. Amorocho in litt. to J. Woody, FWS, 27 February 1990).